1. Field of the Invention
This invention relates to a light beam scanning recording apparatus, wherein a recording material, such as a photosensitive material, is scanned with a light beam, which has been modulated, and a continuous tone image is thereby recorded on the recording material. This invention particularly relates to a light beam scanning recording apparatus, wherein a laser beam produced by a single-longitudinal-mode semiconductor laser is utilized as the recording light beam.
2. Description of the Prior Art
Various light beam scanning recording apparatuses have heretofore been used wherein a light beam, which serves as recording light, is modulated in accordance with an image signal, a recording material (such as a photosensitive material) is scanned with the modulated light beam, and a continuous tone image represented by the image signal is thereby recorded on the recording material. Ordinarily, in such light beam scanning recording apparatuses, semiconductor lasers are utilized as the recording light beam sources. A semiconductor laser has various advantages over a gas laser, or the like, in that the semiconductor laser is small in size, cheap and consumes little power, and in that the laser beam can be modulated directly.
However, it is known that a mode hopping phenomenon occurs with a single-longitudinal-mode semiconductor laser due to a change in the ambient temperature, a change in the drive current, or the like. If the mode hopping phenomenon occurs, the amount of the laser beam produced by the semiconductor laser will fluctuate, or the wavelength of the produced laser beam will fluctuate. If the mode hopping phenomenon occurs with a semiconductor laser, which is employed as a recording light source in a light beam scanning recording apparatus, there will be the risk that nonuniformity in image density occurs in a recorded image due to fluctuations in the amount of the recording light and the dependence of the sensitivity of a recording material upon the wavelength of the recording light.
A method for coping with the mode hopping phenomenon has been proposed in, for example, Japanese Patent Publication No. 59(1984)-9086. With the proposed method, a radio-frequency current is superimposed upon a drive current for a semiconductor laser, and the single-longitudinal-mode semiconductor laser is thereby caused to oscillate in a multiple longitudinal mode.
In cases where a radio-frequency current is superimposed upon a drive current for a semiconductor laser, the degree of modulation (=amplitude/maximum optical output power) of the optical output power of the semiconductor laser by the superimposition of the radio-frequency current has heretofore been set at approximately 100%. However, in cases where such a degree of modulation is to be obtained in a light beam scanning recording apparatus, wherein the laser beam produced by the semiconductor laser is modulated directly, it is necessary to use a semiconductor laser having a very high maximum rating, and therefore the cost of the light beam scanning recording apparatus cannot be kept low. Such problems will hereinbelow be described in detail.
Ordinarily, the image density of a continuous tone image, which is recorded by the light beam scanning recording apparatus, falls within the range of 0 to 3.0. (The term "image density" as used herein means the optical density.) By way of example, it is assumed that an optical output power of the semiconductor laser of 3.3 mW is required in order to record an image with a maximum optical density of approximately 3.0. In cases where a radio-frequency current is superimposed upon the drive current for the semiconductor laser, the image density of the recorded image is determined by the mean value of the optical output power. Therefore, as illustrated in FIG. 4, in order for a mean optical output power of 3.3 mW to be obtained with a degree of modulation of 100%, it becomes necessary that a semiconductor laser having a maximum rating of two times of 3.3 mW, i.e. a maximum rating of 6.6 mW, is employed. On the other hand, semiconductor lasers having only the limited levels of maximum ratings are available commercially. For example, as for semiconductor lasers producing laser beams having wavelengths falling in a 780 nm band, the popularly available semiconductor lasers come in three types of maximum ratings, i.e. 5 mW, 20 mW, and 30 mW. Therefore, in cases where a semiconductor laser having a maximum optical output power of 6.6 mW is necessary, a semiconductor laser having a maximum rating of 20 mW must be selected from those available commercially. As a result, the cost of the light beam scanning recording apparatus cannot be kept low.
As described above, the problems occur with regard to an example of the maximum optical output power of the semiconductor laser, which is required in order to obtain a maximum recorded image density, and maximum ratings of commercially available semiconductor lasers. The same problems also occur in other examples. Thus if a semiconductor laser having a high maximum optical output power is required in order to carry out the superimposition of a radio-frequency current, the cost of the light beam scanning recording apparatus cannot be kept low.